1. Field of the Invention
The present invention relates to an optical data reading apparatus and method for reading data stored in a recording medium such as an optical memory, and in particular to an optical data reading apparatus and method for reading data at a high sensitivity used for a high density optical recording system.
2. Description of the Related Art
As optical memories, read only memories such as compact discs and video discs and rewritable optical memories such as magnetooptical discs are known. Such optical memories are widely used today due to advantages thereof such as large capacity, low cost per bit, and portableness. For the future, such optical memories having a larger capacity and a smaller size are demanded as the society becomes more and more information-oriented.
FIG. 9 shows a representative construction of a conventional optical data reading apparatus designed for reading data stored in a magnetooptical disc 906. As is shown in FIG. 9, the optical data reading apparatus includes a magnet 917, on one of two sides of the magnetooptical disc 906, for applying a magnetic field to the magnetooptical disc 906. On the other side, i.e., a reading side, of the magnetooptical disc 906, the optical data reading apparatus includes a semiconductor laser 901, a collimator lens 902, a beam shaping prism 903, a first beam splitter 904, an objective lens 905, a second beam splitter 907, a .lambda./2 plate 908, a polarization beam splitter 909, condenser lenses 910 and 911 for condensing light carrying an optical signal, high speed PIN photodetectors 912 and 913, a differential amplifier 914, a condenser lens 915 for detecting a tracking error/focus error, and a photodetector 916 for detecting a tracking error/focus error.
Optical data is read in the following manner by such an optical disc reading apparatus.
Laser light emitted by the semiconductor laser 901 is collimated by the collimator lens 902, and then shaped into a circular beam by the beam shaping prism 903. The laser light emitted by the semiconductor 901 is linearly polarized light. Such linearly polarized laser light is transmitted through the first beam splitter 904, condensed by the objective lens 905, and then radiated to the magnetooptical disc 906. Data is digitally recorded in the magnetooptical disc 906 by magnetizing the magnetooptical disc 906 perpendicularly to two surfaces thereof. When the radiated laser light is reflected by the magnetooptical disc 906, the plane of polarization of the laser light is rotated as a result of the Kerr effect in accordance with the data stored in the magnetooptical disc 906. A rotation direction of the plane of polarization in accordance with data "1" is opposite to a rotation direction thereof in accordance with data "0". Using this principle, the data are read by detecting in which direction the plane of polarization is rotated.
After being reflected by the magnetooptical disc 906, the laser light having optical signal corresponding to data "1" or "0" is turned at 90.degree. by the first beam splitter 904 and then divided into a first component and a second component by the second beam splitter 907. The first component is transmitted through the condenser lens 915, and then converted by the photodetector 916 into an electric signal used for detecting a tracking error/focus error. A plane of polarization of the second component is rotated at 45.degree. by the .lambda./2 plate 908, and thus resultant optical signal is used for data detection.
FIG. 10 shows two components included in the laser light immediately after transmitting through the .lambda./2 plate 908. As is shown in FIG. 10, the laser light includes a component 1001 (corresponding to the second component) obtained by rotating the plane of polarization of the laser light by the magnetooptical disc 906 and a component 1002 having a plane of polarization thereof not being rotated. The component 1002 includes noise obtained through reflection by surfaces of the parts of the optical data reading apparatus other than the magnetooptical disc 906. The components 1001 and 1002 are superimposed on each other. The laser light including the components 1001 and 1002 is divided into an s-wave component and a p-wave component when being incident on the polarization beam splitter 909. The p-wave component is transmitted straight through the polarization beam splitter 909, whereas the s-wave component is turned at 90.degree. by the polarization beam splitter 909.
The p-wave component is condensed by the high speed condenser lens 911 to the high speed PIN photodetector 913, where the p-wave component is converted into an electric signal. The s-wave component is condensed by the condenser lens 910 to the high speed PIN photodetector 912, where the s-wave component is converted into an electric signal. The electric signals outputted from the high speed PIN photodetectors 912 and 913 are amplified by the differential amplifier 914. The differential amplification is performed in order to eliminate the electric signal obtained from the component 1002 and thus to detect only the electric signal obtained from the component 1001 as an electric output.
Practically, in the case of the component 1001 shown in FIG. 10, the p-wave component is larger than the s-wave component. Accordingly, the electric signal obtained by the differential amplification, i.e., the (p-wave component)-(s-wave component) has a positive value. In a case where the plane of polarization of the component 1001 is rotated in the opposite direction by the magnetooptical disc 906, i.e., in a case where the optical data is recorded in the magnetooptical disc 906 by magnetizing the magnetooptical disc 906 oppositely to the case shown in FIG. 10, the s-wave component is larger than the p-wave component. In such a case, the electric signal obtained by the differential amplification has a negative value. In the component 1002, the p-wave component and the s-wave component have an identical level with each other in either case. By this principle, only the electric signal obtained from the component 1001 is detected as an electric output by the differential amplifier 914, and thus the data "1" or "0" stored in the magnetooptical disc 906 is restored.
In the above example, differential amplification is used. In all types of conventional optical data reading apparatuses including an apparatus for a magnetooptical disc, a direct detecting method for directly detecting the intensity of light is used for signal detection.
For optical recording mediums such as an optical memory for a magnetooptical disc, a larger capacity and a higher recording speed are demanded. In order to fulfill such demands, a magnetooptical disc having a higher density recording medium and a higher rotation speed is required. In correspondence with such development in the density and the rotation speed, a recording area allocated for one bit, i.e., one recording unit is further reduced. As a result, the intensity of the light reflected by the recording medium (i.e., signal light) is lowered, and the pulse width of the signal light per bit is decreased. In other words, an amount of energy of the signal light per bit is reduced. Under these circumstances, the power level of the signal light becomes close to the temporal noise level of the photodetector. For this reason, a conventional optical data reading apparatus which uses light having a wavelength in a range around 780 nm has a problem in that data stored in a disc having a high density of 0.7 Mbits/mm.sup.2 cannot be read with high sensitivity.
As one solution of this problem, use of laser light having a shorter wavelength has actively been studied. However, a laser for emitting laser light having a short wavelength which is desirable for an optical memory having a largest memory available is not put into practice in time with the development of such an optical memory. Even if a laser for emitting light having a desirably short wavelength is developed, an optical memory having a still larger capacity which can be operated using such a laser is demanded. In consideration of these matters, this approach also has a problem in that a highly sensitive detecting mechanism is required.
As is mentioned above, the conventional optical data reading apparatuses employ a direct detecting method in which the level of an optical output is directly detected by a photodetector. Such a method cannot cope with future increases in memory capacity, density and recording speed.